Military rifles and machine guns are capable of consuming large quantities of ammunition in a short time, and it is desirable to supply these quantities to the rifle without frequent interruptions for reloading. There have been many attempts to provide military forces with a small arms improvement for an advantage in combat. High capacity firing devices provide a dominant support tool in the battlefield. There is a significant need for our military to have the option of a device that can reliably provide a 150-round capacity to all of the weaponry firing from a NATO standard, STANAG 4179, magazine. Belt-fed machine guns are effective for sustained fire without reloading, except that a belt-fed machine gun is by nature a more complicated and heavy system because of the additional apparatus required to move the belt, extract the rounds from the belt, and chamber the cartridge. This is in addition to the weight of the apparatus attached to the gun to hold the ammunition belt and the steel links holding the belt together.
A more modern machine gun capable of feeding both belts and magazines performs less reliably than a gun optimized for one or the other. This is because of the different dynamics when feeding a cartridge from a simple magazine compared to the tension required to draw in a belt. A significant tension is required to lift the long belt the pitch distance of the rounds in the belt, and to strip the cartridge from the link to which is holding it. This performance difference is most easily noticed by the increase in recoil and cycle rate when firing from the magazine in a rifle optimized with adequate cyclic energy to feed a belt.
Belts can become dirty or angled in the field, leading to malfunctions. Typical stick or box magazines having 20-40 cartridges in a vertical stack, typically two columns side-by-side, are more protective of the ammunition, but require much more frequent interruptions for extraction of the spent magazine and insertion of a fresh magazine. These can be depleted after as little as two seconds of sustained firing. Conventional magazines have a spring located in the bottom of the magazine. This spring must have enough force to efficiently force the last round into the feeding position in the short time allowed, even when the spring is in its most relaxed condition. This causes the spring to be larger if there is more ammunition to be lifted. This compounds the problem with vertical height issues as it is of key importance that the shooter remain as low as possible when in combat. Longer stick magazines can provide greater capacity, except that the added length forces a prone shooter to hold the rifle higher above the ground, making him more vulnerable to enemy fire.
Drum magazines such as employed in the Thompson submachine gun, store cartridges in a cylindrical body that permits a larger capacity. The Thompson gun lacks a protected magazine well, so that the periphery of an installed drum is nearly coincident with the firearm bore and allows the cartridges to be easily picked from the drum and directly inserted into the chamber. The cartridges were contained within a spiral and were pushed along the spiral path until exiting the opening at the top of the drum. It was found that this system could reliably feed only about 50 rounds before the friction drag from the cartridges against the spiral would result in failures. It was also deemed difficult because the re-loading process required that the drum be opened and ammunition be placed into separate loading channels.
A dual drum magazine, such as disclosed in U.S. Pat. No. 4,658,700 to Sullivan, provides substantial capacity (upwards of 100 cartridges, depending on caliber) without significant height by positioning one drum one on each side of a central column. The column is essentially the upper portion of a stick magazine that inserts into a rifle's magazine well, with the drums on either side feeding the column. The inside stack of cartridges in each drum is driven by a sprocket, the outside row of cartridges is driven by the inside row of cartridges. This creates great frictional forces against the wall because of the cartridge to cartridge contact causes an outward push along with the rotational force driving the second row of cartridges. This is compounded when the double row of cartridges, of which only one has direct power and the other is powered by path of least resistance, try to leave the drum and are driven by a cam blade into a single stack. The cartridges then change direction by 90 degrees and join in a parallel stack. For reliability of feeding, it is desirable to have a very strong force, which is readily achieved when the magazine is full and the feed springs fully compressed, but more critical when the last rounds are being fed. Because of high friction drag design, introduction of dirt or dust may increase friction to unacceptable levels, and impair reliable feeding. Moreover, loading of the magazine is difficult as it becomes full, due to the force of the feed springs that resist cartridge insertion. This also may cause failures of speed loader devices.
Another drum magazine is disclosed in U.S. Pat. No. 4,384,508 to Sullivan et. al, which employs a single drum and a sprocket feed system, and is incorporated herein by reference. A set or series of concentric sprockets each carry a single ring of cartridges, and rotate independently to provide lower friction feeding of one ring of cartridges at a time. This system allows the cartridges to be individually nested with less overall friction and free from significant drag until picked from the sprocket. This was designed in conjunction with the firearm for which it was to operate. Consequently, this firearm has no supportive magazine well. The drum diameter is nearly coincident with the firearm bore and allows the cartridges to be easily picked from the drum and directly inserted into the chamber. This feeds the cartridges outward at a port that extends radially, not tangentially from the cylindrical outer wall, and is unsuitable for use with magazine-fed rifles due to the inadequate column length, and the substantial downward extension that magazine would present. A major problem with this system is that it was powered by a strong spring that is difficult to counter during reloading. While internally effective with certain limitations, it is suited for rifles specially built to accommodate it, and not for rifles with magazine wells that are designed to protect the conventional stick magazines that they are designed to accept.
The present invention overcomes the limitations of the prior art by providing a detachable magazine for storing and delivering ammunition to a firearm having a magazine well. The magazine has a body with a column adapted for insertion into the magazine well. The column has a passage for transmitting ammunition to the firearm. The body includes a drum housing defining a substantially cylindrical chamber communicating with the column passage. A first sprocket element is rotatably received in the chamber and has a serrated periphery, with each serration adapted to receive an ammunition cartridge. A second sprocket element is rotatably received in the chamber, and is concentric with the first sprocket element, having a serrated periphery with each serration adapted to receive an ammunition cartridge. A spring element is connected to at least one of the sprocket elements, and operates to rotatably bias the sprocket elements to transmit cartridges from the drum chamber to the column passage. The magazine may have a pair of drums, and the sprockets may be tapered, to facilitate feeding of tapered cartridges. The magazine may include a counter to indicate the quantity of ammunition consumed or remaining. The magazine may include the ability to power and de-power spring motor.